1. Field of the Invention
The present invention relates to dome-style surveillance camera systems and, more particularly, to dome-style surveillance camera systems that can be used outdoors.
2. Description of the Related Art
Surveillance camera systems are commonly used by retail stores, banks, casinos and other organizations to monitor activities within a given area. The cameras are often provided with the capability to pan and tilt in order to acquire images over a wide domain. The tilt of the camera generally refers to the pivoting of the camera about a horizontal axis that is parallel to the floor, such that the lens of the camera may tilt between an upwardly pointing position and a downwardly pointing position. The pan of the camera refers to the rotation of the camera about a vertical axis that is perpendicular to the floor, such that the lens may scan from side to side. The cameras may also be able to zoom in order to reduce or enlarge the field of view. Oftentimes, each camera is linked to video display units in a security surveillance room with surveillance personnel monitoring the multiple video display units.
Surveillance cameras may be mounted within a hemispherical dome window constructed of a material that is transparent when viewing outward and only partially transparent when viewing inward to inhibit unauthorized individuals from determining the area being viewed by the camera. Similarly to sunglasses, the window may be tinted or provided with a thin metallized layer. To further inhibit unauthorized individuals from seeing the position of the camera, the camera is typically encased in a “covert liner”, which is generally formed of an opaque matte black material and attached to the pan stage in order to pan with the camera. The covert liner may conform to and be slightly offset from the inside surface of the window. The liner includes a slot through which the camera may view. The slot may extend 90° or more from the apex to the horizon or beyond.
For outdoor applications, the dome window should be of a one-piece, i.e., unitary or monolithic, construction so that there are no seams through which moisture or dirt may pass and thereby possibly contaminate the camera. The dome window is typically formed of a molded plastic material. In order that the plastic dome can be easily removed from the mold without destroying the mold, the inner cavity of the dome should have a width that is constantly increasing, or at least lacking any decrease, along a vertical direction. Thus, the extent of the curvature of the dome window may be limited to 180°, i.e., the curvature of a hemisphere.
The open end of the hemispherical dome window is typically fixedly mounted in a horizontal orientation to some form of overhead mounting apparatus. A tight seal may be provided between the dome window and the overhead mounting apparatus to ensure that no dirt or moisture can enter the dome. Thus, the dome window is typically fixed relative to the overhead mounting apparatus and is not subject to the panning, tilting and zooming movement of the camera contained therein. Consequently, the positioning of the camera via the panning, tilting and zooming may enable the camera to view through any area of the dome window.
In order for the camera to view in horizontal directions without being obstructed by the ceiling or overhead mounting apparatus, the camera is often mounted such that the camera's tilt axis is significantly offset below the geometric center of the hemispherical dome window. The dome window diameter is therefore determined by the camera rotational diameter plus twice the tilt axis offset distance. This larger dome window occupies more space and is more expensive to produce.
With this offset, the line of sight of the camera may be non-perpendicular to the concave inner surface of the dome window at the point where the line of sight intersects the concave inner surface. This may result in refractive distortion of the images received by the camera, particularly in the upper range of camera tilt positions. Increases in offset and dome diameter may worsen the distortion. The refractive distortion may combine with autofocus lens algorithms to result in ghosting, loss of horizontal feature darkness value, and vertical variation of picture quality.
It would be further desirable for the camera to be able to view in directions above the horizon. However, viewing above the horizon would require the tilt axis of the camera to be lowered even farther away from the geometric center of the hemispherical dome window. This would exacerbate problems with refractive distortion. Refractive distortion may be particularly troublesome when viewing in directions above the horizon because the curvature of the dome window slopes slightly outwardly away from the camera.
What is needed in the art is a surveillance camera assembly including a dome window that enables the camera to view in a horizontal direction without obstruction and without requiring the tilt axis of the camera to be positioned below the geometric center of the hemispherical dome window. What is also needed in the art is a surveillance camera assembly including a dome window that enables the camera to view in a slightly upward direction above the horizontal direction.